Alder

Location and Geology

The Alder field is situated in Block 15/29a of the UK Continental Shelf, in the central North Sea. It is approximately 100 miles away from the Scottish coastline and 37 miles from the UK/Norway median line.
The field lies in a water depth of 492 feet (150 meters).
It is an Upper Jurassic reservoir with high-pressure and high-temperature (HP/HT) characteristics, featuring a wellhead shut-in pressure of 9,440 psi and a reservoir temperature of 305°F (151°C).

The Alder field was discovered in 1975 with the drilling of the 15/29a-2 well.
Four appraisal wells (15/29a-3, 15/29a-5, 15/29a-7, and 15/29a-8) were drilled in 1988, 1990, 1993, and 1994, respectively.

The project is operated by Chevron North Sea Ltd., which holds a 73.684% interest. ConocoPhillips (UK) Ltd. is the co-venturer with a 26.316% interest.

Initially, Chevron considered developing the field using a floating production, storage, and offloading (FPSO) solution. However, a subsea development plan was preferred in 2010 due to greater environmental risks associated with the FPSO option.
The field was developed by tying back to a single subsea well connected to the Britannia bridge-linked platform (BLP) via a 28 km (17 miles) long production flow line. The second well is planned to be drilled in a later phase of the field development.
The production manifold connects the wellheads to a 10-inch and 16-inch pipe-in-pipe system, which runs parallel to an umbilical line. The pipeline uses the existing riser and J-tube on the Britannia BLP.

A conventional semi-submersible drilling rig was used for drilling the wells, employing riser-less drilling techniques for the top-hole sections and low-toxicity oil-based mud (LTOBM) for the lower sections.
A dynamically positioned (DP) reel-lay vessel was used for laying the pipes, with mechanical plough or jetting for trenching and burial of the flowline and umbilical.
The topsides module, weighing 800 metric tons, was built in a Newcastle shipyard and installed on the Britannia BLP in June 2015.

The Alder field is estimated to contain approximately 4,820 million standard cubic meters (scm) of gas. It is designed to produce 110 million cubic feet of natural gas and 14,000 barrels of condensate per day.
Production commenced in November 2016. The produced gas is exported through the Scottish Area Gas Evacuation (SAGE) pipeline to St Fergus, while the condensate is transported via the Forties pipeline to the Grangemouth oil terminal in the UK.

OneSubsea UK supplied the vertical monobore subsea trees and wellheads for the Alder project.
Aker Solutions was contracted for the design, manufacture, and supply of the subsea control system, including hydraulic and electrical equipment.

About 75% of the development work for Alder was executed in the UK, involving various locations such as Aberdeen, Invergordon, Leeds, and Newcastle.
The project's environmental considerations led to the preference for a subsea development plan over the FPSO solution to mitigate greater environmental risks.